Elevated Temperature Tensile and Creep Strength

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FIGURE A3.12 Effect of alternating stresses with and without corrosion

Tensile tests are performed at elevated temperatures to characterize the material’s yield and ultimate tensile properties at potential use temperatures above room temperature. A heating chamber is combined with a conventional tensile testing machine, and special strain measuring extensometers are used that are capable of withstanding the test temperatures. Generally, as temperature increases, yield and ultimate strengths decrease, and ductility increases.

FIGUREA3.13 Creep time versus elongation curves at a given temperature

Creep is defined as the time-dependent deformation of a material that occurs under load at elevated temperatures. The test is performed by holding a specimen, similar in configuration to a tensile specimen, at a uniform temperature and a constant load (usually using a dead weight) and allowing the specimen to gradually elongate to ultimate failure. The practice is defined in ASTM Specification E 139. The simplest test method records only the applied stress (based on original test specimen cross section), time to failure, and total elongation at failure. This is called a stress rupture test. If periodic measurements of strain accumulation versus test duration are also taken, the test is referred to as a creep-rupture test.

A representation of typical creep strain-versus-time data is shown in Fig. A3.13. Three stages of creep behavior are exhibited. Upon initial loading, instantaneous straining occurs. Almost immediately, the rate of creep strain accumulation (creep rate) is high but continuously decreasing. The test then progresses into a phase where the strain rate slows and becomes fairly constant for a long period of time. Finally, with decreasing load-bearing cross section of the specimen due to specimen stretching and necking, applied stress begins to increase steadily, as does the creep rate, until failure occurs. These three regions are termed the primary, secondary, and tertiary stages of creep. The intent of safe design practice is to avoid the third stage, where strain accumulations are rapid and material behavior less predictable.

FIGURE A3.14 Typical stress-to-rupture curves

After accumulating a number of rupture data points (i.e., time to failure of a metal at various applied stresses), the data is generally represented as a stress rupture curve (Fig. A3.14). Each curve represents the time to failure at various applied stresses, at a given test temperature. Another useful property that can be measured in these tests is the creep rate during the second stage of creep, for a given applied stress and temperature. This, along with time to onset of the tertiary creep stage, are useful properties
to the design engineer and are used in establishing allowable design tension stresses in design codes.

Metals that experience creep will accumulate a progressively larger amount of microscopic damage to the structure of the material. Damage is first observed microscopically as small cavities, or voids, that begin appearing in the grain boundaries of the metal, particularly at triple points (i.e., where three grains come together). Further progression of damage entails formation of more voids along many of the adjacent grain boundaries, until ultimately they link together to form grain boundary microcracks. With more time, these form larger macrocracks that lead to ultimate failure of the metal component. The determination of a metal’s degree of creep damage, and its consequence on the continued safe operation of the component, has developed into a sophisticated science referred to as component condition assessment, or estimation of remaining life. This will be addressed in more detail later in the chapter.

A practice essentially identical to cyclic fraction life summation used in fatigue design can be employed in material creep analysis to estimate the percentage of creep life expended. Here the individual life fraction corresponds to the amount of time a component spends at a given stress and temperature, compared to the total time to failure given on the stress rupture curve for the same applied stress and temperature. All of these fractions for all the operating conditions are then added together, and compared to an appropriate design limit (1.0 or less).